Prostaglandin ester analogues of the following Formula I-2
wherein
is
 is a single or double bond; R2 is a single bond or a C1-4-alkylene or —CH2O—; and R3 is a C1-7-alkyl or an aryl or an aralkyl, each of which is unsubstituted or substituted by a C1-4-alkyl, a halogen or a trihalomethyl; and R4 is C1-7-alkyl, such as, Latanoprost, Isopropyl unoprostone, Isopropyl cloprostenol, Travoprost and Tafluprost have been used in the management of open-angle glaucoma. The Prostaglandin ester analogues of Formula I-2 have been shown to have significantly greater hypotensive potency than the parent compound, presumably as a result of their more effective penetration through the cornea. They reduce intra-ocular pressure by enhancing uveoscleral outflow, and may also have some effect on trabecular meshwork as well.
As shown in the following Scheme A:
most of the Prostaglandin ester analogues of Formula I-2 disclosed in the prior art, such as in WO02096898, EP1886992, EP2143712, JP2012246301, U.S. Pat. No. 6,720,438, US2008033176, WO2010097672, and U.S. Pat. No. 7,582,779 were obtained by first synthesizing a Lactone VIII, wherein
is
or is a protective group of carbonyl group; P1 and P2 are protective groups for the hydroxyl groups;  is a single or double bond; R2 is a single bond or a C1-4-alkylene or —CH2O—; and R3 is a C1-7-alkyl or an aryl or an aralkyl, each of which is unsubstituted or substituted by a C1-4-alkyl, a halogen or a trihalomethyl and then conducting a semi-reduction of the Lactone to get a Lactol VII which was underwent a Wittig reaction to produce a C5˜C6 configuration cis-olefin of Formula IV-1, which was then converted to Prostaglandin ester analogues of Formula I-2. No matter which kind of Wittig reagents or solvents were used or what was the temperature for the Wittig reaction, it was inevitable that about 2˜10% 5,6-trans isomer of the compound of Formula IV-1 would be produced. If the starting material already contained a trace amount of an isomer (e.g., 15β-isomer or enantiomer), the resultant compound of Formula IV-1 would contain the corresponding isomer.
As shown in the following Scheme B:
some of the syntheses of Prostaglandin ester analogues of Formula I-2, for example, latanoprost disclosed in the prior art, such as WO02090324 and US2009259058, were involved a conjugate addition of a cyclopentenone VI′ with a ω-side chain unit IX′ to obtain a cyclopentanone V-2′, which was then underwent a 9-keto reduction to get a protected latanoprost IV-2′. Nevertheless, such a conjugate addition could not avoid the generation of a trace amount of 8β-isomer and 12α-isomer, nor could it avoid the generation of a trace amount of 9β-isomer. In addition, commercially available cyclopentenone VI′ and ω-side chain unit IX′ very likely contain trace amount of enantiomers and as a result, in the reaction of Scheme B, the 15β-isomer of latanoprost would be produced. Moreover, commercially available cyclopentenone VI′ could contain a trace amount of 5,6-trans isomer and as a result, in the reaction of Scheme B, the 5,6-trans isomer of the latanoprost would be produced.
As shown in the following Scheme C:
WO2011008756 discloses the synthesis of Travoprost by a ring-closing reaction with the cyclopentane ring at the presence of a Grubbs catalyst, and conducting deprotection and ring-opening reactions to obtain Travoprost. Although WO2011008756 mentions that the ring-closing reaction would obtain an olefin with a configuration of “cis” at C5˜C6, upon a study made by the inventor, the ring-closing reaction of WO2011008756 is still involved the generation of a certain amount of 5,6-trans isomer.
Latanoprost, Isopropyl unoprostone, Travoprost and Tafluprost all are not solids, and their free acid forms are not solids, either. Even in all the processes shown in Schemes A to C, none of the intermediates with the necessary chiro centers and olefins being established could be crystallized. Consequently, it is unlikely to purify these Prostaglandin analogues or intermediates by crystallization to remove the isomers. Therefore, it was almost impossible to obtain any isomer free Prostaglandin ester analogues of Formula I-2 in an oil form through common purification technology.
While WO02096898 and WO2011005505 disclose methods for removing the 5,6-trans isomer and 15-isomer of latanoprost by preparative HPLC and WO201109599 discloses removing the isomer of latanoprost acid by reverse phase preparative HPLC, these purification methods by utilization preparative HPLC to remove the isomer are costly and not suitable for mass productions.
Given the above, commercially available Prostaglandin ester analogues of Formula I-2, either as active pharmaceutical ingredients or in the form of formulation products contain a certain amount of isomers, particularly 5,6-trans isomer. For medicine safety and reducing production cost, the present invention provides a simpler process for producing isomer free Prostaglandin ester analogues of Formula I-2 where unwanted isomers, particularly 5,6-trans isomers can be effectively and easily removed during the processes.